1. Field of Endeavor
The present invention relates to filters and more particularly to fluid filters.
2. State of Technology
There are four broad categories of filtration systems. The first, and most common, employs a matrix-type filter. The matrix-type filter is made of cellulose, polyurethane, and other materials. The matrix-type filter employs various layers of a material that capture particles by providing a tortuous path with a mean physical porosity smaller than the target particles. The biggest drawback of this type of filter is the difficulty in recovering particles once they are trapped. In addition, any single filter can only be used to filter particles above a certain size threshold and cannot be dynamically varied.
The second category can be labeled as physical, non-matrix type filters such as Anapore. These materials provide a planar structure (such as ceramic or silicon) with relatively uniform holes through which fluid can pass, but particles become trapped. Recovery from these filters is possible since the particles do not become embedded in the filter material, as happens with the matrix-type filters. These filters, however, tend to be brittle and have a predetermined particle filter limit (determined by the hole size). In addition, the two dimensional nature of these filters and the need to place them orthogonal to the inlet flow, places practical design constraints on the packaged filter system. These include difficulty in creating a uniform seal around the filter, flow rate limitations dictated by the filter area and pore size, and constraints on the volume of fluid used to recover particles from the filter surface.
The third category of filter uses the dielectric properties of the particles coupled with a magnetic or electrical capture field. Magnetic separation is the most commonly used method in bioanalytical processes. There are several disadvantages to using magnetic beads. Filtration of biologicals such as bacteria, spores, and cells is difficult or impossible since their attraction in electrical or magnetic fields is fairly weak. In addition, this method requires instrumentation (magnets, electrodes, etc.) and design of associated EM fields for capturing particles with a given dielectric property. This precludes the use of many materials (polymers, etc.) that are commonly doped with fluorescent compounds for multiplex analysis in optical based detection modalities such as flow cytometry.
The fourth category is a fixed-gap type filter. This category of filter typically uses precision machining or micromachining (i.e., silicon etching) methods to create very uniform barriers for trapping particles. The advantage of this method is the ability to create highly reproducible filters with small volume sizes that can be used to both capture and recover particles. The principal disadvantage of this category of filter is the inability to dynamically change the barrier size. In addition, integration and packaging of microfluidic devices is still challenging.